1. Field of the Invention
This invention relates generally to power generation systems and, more particularly, this invention relates to power generation systems utilizing electrochemical cells and methods for improving performance therein.
2. Description of the Related Art
Electrochemical cells utilizing consumable metal anodes are well known. Such cells are described in detail in numerous patents and publications, including Rowley U.S. Pat. No. 3,791,871 (Feb. 12, 1974), Tsai et al U.S. Pat. No. 3,976,509 (Aug. 24, 1976), and Galbraith U.S. Pat. No. 4,528,248 (Jul. 9, 1985), the respective disclosures of which are incorporated herein by reference.
The cell disclosed in Rowley U.S. Pat. No. 3,791,871 typifies prior electrochemical cells utilizing a consumable metal anode which is highly reactive with water and spaced from a cathode by an electrically insulating film formed on the anode in the presence of water. The anode and cathode are in contact with an aqueous electrolyte during cell operation. In the cell of the Rowley patent, the anode comprises an elemental alkali metal such as sodium or lithium, and the electrolyte comprises an aqueous solution of the hydroxide of the anodic metal produced by the electrochemical reaction of the anodic metal with water.
The operation of such cells involves the following reactions which, for illustrative purposes, utilize lithium as the active material of the consumable metal anode, and aqueous lithium hydroxide as the electrolyte.
A. Anode Reaction
Electrochemical Dissolution EQU Li.fwdarw.Li.sup.+.sub.(aq) +e.sup.- ( 1)
Formation of Insulating Film on Anode EQU Li.sup.+.sub.(aq) +OH.sup.-.sub.(aq) .fwdarw.LiOH.sub.(aq) ( 2) EQU LiOH.sub.(aq) .fwdarw.LiOH.sub.(s) ( 3)
Parasitic Corrosion Reaction EQU Li+H.sub.2 O.fwdarw.LiOH.sub.(aq) +1/2H.sub.2(g) ( 4)
B. Cathode Reaction
Reduction of Water EQU H.sub.2 O+e.sup.- .fwdarw.OH.sup.- +1/2H.sub.2(g) ( 5)
(aq) represents an ion dissolved in water and (s) represents a solid salt.
Reactions (1) and (5) are necessary for the generation of electricity. Reactions (2) and (3) serve to produce a porous insulating film which forms on the anode and protects it. Reaction (4) is a parasitic corrosion reaction which consumes the active anodic material and produces hydrogen gas but generates no useful current.
The anode of the Rowley patent is coated with a thin film of a nonreactive, partially water soluble material which is not electrically conductive. Preferably, the film is the natural hydrated oxide which forms on the metal surface as it is exposed to humid air. However, other suitable water soluble insulators may serve as the film. The film is porous and allows transport of aqueous electrolyte to the anode and transport of reaction products away from the anode.
The electrolyte of the cell disclosed in the Rowley patent is formed by the electrochemical reaction of water and the anodic metal. Thus, in the Rowley cell, water is introduced to the cell at a restricted rate and brought into direct contact with both the cathode and anode. The water dissolves a portion of the soluble film on the anode, resulting in the production of a hydrated hydroxide of the anode material, plus heat. As the reaction proceeds, useful electrical power is produced.
The anode and the cathode are not in direct contact with each other, but circuit connections are made at each of the cathode and anode for drawing electrical power from the cell.
The electrolyte is preferably a solution of the hydroxide of the anodic metal since such hydroxide is naturally formed during operation of the cell and hence automatically regenerates the electrolyte during operation.
The alkali metal of the Rowley anode is highly reactive with water. This reactivity decreases (at a given temperature) as the concentration of metal hydroxide in the electrolyte increases. Optimally (at typical operating temperatures), the concentration of lithium hydroxide in the electrolyte is maintained at about 4.2-4.5 molar. As the lithium hydroxide concentration in the cell rises, the rate of power generation from the cell correspondingly diminshes, and passivation of the anode can occur if the electrolyte becomes saturated with lithium hydroxide.
Thus, in these electrochemical cells, relatively high concentrations of the consumable metal hydroxide generally must be avoided to maintain a desired level of power output at typical operating temperatures. Therefore, steps must be taken to maintain the reactive metal hydroxide concentration in the electrolyte at a level at which useful electrical current is produced.
One solution to the problem of too great a concentration of the reactive metal hydroxide in the electrolyte is the continuous expulsion of a fraction of the electrolyte stream into the surrounding environment and the simultaneous injection of a similar flow rate of fresh water into the electrolyte. If the stream input and output are kept balanced and prorated by metal hydroxide production, this technique is effective. However, the technique has several disadvantages. Firstly, the motion of the inlet and outlet flow streams results in significant noise levels and in some applications the noise generated may exceed desired and/or tolerable noise limits. Secondly, the technique requires a continuous source of fresh feed water. For non-marine applications, there is no such ready source of inlet water and even if such inlet water were carried on board, its weight would in most cases, be prohibitive. Accordingly, all such closed loop electrochemical cells require some form of "electrolyte management", i.e., the removal of the reactive metal hydroxide from the circulated electrolyte.
The use of simple acids, such as phosphoric acid, hydrogen fluoride, etc., as electrolyte management agents for closed loop electrochemical cells, wherein the simple acid acts as a precipitant for the reactive metal hydroxide, generally suffers from the relatively great overhead weight burden imposed on the cell per gram of reactive metal hydroxide removed from the circulated electrolyte. Conventional precipitation techniques generally require large amounts of consumable reactants and result in the formation of large amounts of reaction products. The burden of carrying precipitating agents and the subsequently formed reaction products seriously reduce the specific energy and specific power of such power generation systems. Also, the extreme toxicity, volatility and dangerous propensities exhibited by some simple acids, such as hydrogen fluoride, make these materials unattractive as electrolyte management agents.
Electrolyte management may present a significant burden to reactive metal/aqueous electrolyte electrochemical cells.